Studies in Avian Biology 07

Population Ecology of the
Dipper (C&c/us mexicunus)
in the Front Range
of Colorado

FRANK

E. PRICE

and CARL

E. BOCK

DEPARTMENT
OF ENVIRONMENTAL,
POPULATION
AND ORGANISMAL
BIOLOGY
UNIVERSITY OF COLORADO
BOULDER, COLORADO

Studies in Avian Biology No. 7
A PUBLICATION

OF THE COOPER ORNITHOLOGICAL

SOCIETY

Cover Photograph: Dipper, by Don Bleitz, Bleitz Wildlife Foundation,
Hollywood, California

STUDIES IN AVIAN BIOLOGY
Edited by
RALPH J. RAITT
with the assistanceof
JEAN P. THOMPSON
at the
Department of Biology
New Mexico State University
Las Cruces. New Mexico 88003
EDITORIAL
Joseph R. Jehl, Jr.

ADVISORY BOARD

Frank A. Pitelka

Dennis M. Power

Studies in Aviun Biology, as successorto PaciJc Coast Avifuunu, is a series
of works too long for The Condor, published at irregular intervals by the Cooper
Ornithological Society. Manuscripts for consideration should be submitted to
the Editor at the above address. Style and format shouldfollow those of previous
issues.

Price: $9.00 including postage and handling. All orders cash in advance; make
checks payable to Cooper Ornithological Society. Send orders to Allen Press,
Inc., P.O. Box 368, Lawrence, Kansas 66044. For information on other publications of the Society, see recent issuesof The Condor.
Current address of Frank E. Price: Biology Department, Hamilton College,

Clinton, New York 13323.
Library of CongressCatalog Card Number 83-73016
Printed by the Allen Press, Inc., Lawrence, Kansas 66044
Issued November 8, 1983
Copyright by Cooper Ornithological

ii

Society, 1983


CONTENTS
INTRODUCTION .......................................
STUDY AREAS ........................................
METHODS ............................................
Maps and Measurements ............................
Banding ...........................................
Determination of Sex and Age .......................
Censusing .........................................
Determination of Territory Boundaries ...............
Measures of Habitat Quality .........................
Statistical Analyses .................................
ANNUAL CYCLE IN THE COLORADO FRONT RANGE
POPULATION MOVEMENT ...............................
Seasonal Movement in Altitude ......................
Postbreeding Movement of Adults ...................
Dispersal of Juveniles ...............................
Movement in Winter ...............................
Movement Between Drainages .......................
Homing by Adult Dippers ...........................

Discussion of Movement ............................
POPULATION DENSITY AND DISPERSION ..................
Seasonal Trends in Population Density ...............
Environmental
Factors Affecting Dispersion ...........
Social Factors Affecting Dispersion ...................
Discussion of Density and Dispersion ................
SURVIVAL AND PRODUCTIVITY ..........................
Survival and Mortality ..............................
Productivity and Recruitment
.......................
Effect of Stochastic Events on Survival and Productivity
Discussion of Survival and Productivity
..............
GENERAL DISCUSSION AND CONCLUSIONS ................
Front Range Dipper Populations .....................
Population Regulation ..............................
ACKNOWLEDGMENTS. ..................................
LITERATURE CITED ....................................

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TABLES
Table

Table
Table
Table
Table
Table
Table

1.
2.
3.
4.
5.
6.
7.

Table 8.
Table 9.
Table
Table
Table
Table
Table
Table
Table

10.
1 1.
12.
3.
4.

5.
16.

Table 17.
Table 18.
Table 19.

Comparison of habitat quality and population density of study areas ............
Listofvariablenames
.....................................................
Contintentality indices and elevations of studies of Dipper populations. .........
Examplesofwintermovements..
...........................................
Number (%) of monthly censuses with random dispersion of Dippers ...........
Multiple correlations of environmental variables with dispersion in each season
Relative importance of variables affecting dispersion on Boulder Creek in different
seasons ...................................................................
Relative importance of variables affecting dispersion on South Boulder Creek in
different seasons ...........................................................
Summary of relative importance of variables affecting dispersion on Boulder and
South Boulder Creeks.. ............
.......................................
Stepwise correlation of female territory size with six variables ..................
Number of breeding attempts and evidence for population surplus ..............
Estimated survival rates of adult and juvenile Dippers. ........................
Relative loss of Dippers from study areas, summer vs. winter ..................
Productivity of the Boulder area Dipper population ...........................
Reported clutch sizes and fledging success for the Cinclidae ....................
Stepwise correlation of eight variables with number of fledglings per brood (197 l1973) ....................................................................
Multiple and stepwise correlations of grouped variables with number of fledglings

per brood (1971-1973).
....................................................
Multiple and stepwise correlations of grouped variables with number of fledglings
per brood for subsets of data. ...
..........................................
Summary of major factors affecting the Boulder area Dipper population .........

9
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FIGURES
Figure
Figure

Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure

1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.

Figure
Figure 1’

Figure
Figure 1
Figure 19.
Figure 20.

Genera1 map of study area. .................................................
Map of South Boulder Creek study area. .....................................
Map of Boulder Creek study area. ...........................................
Variation of environmental factors in Boulder, Colorado. ......................
Timing and number of clutches being incubated, 1971-1973
..................
Number of banded birds arriving and departing study areas ....................
Mean number of Dippers moving more than 1.6 km on study areas. ............
Numbers of 1971 and 1972 breeding birds present on study areas after breeding.
Boulder Creek food samples ........................................
......
South Boulder Creek food samples ..............
..........................
Home ranges and interactions of wintering Dippers ...........................
Densities observed on Boulder Creek ........................................
Densities observed on South Boulder Creek ..................................
Breeding territories, 1971-1973, and 1973 breeding season food on South Boulder
Creek ...................................................................
Breeding territories, 1971-1973, and 1973 breeding season food on Boulder
Creek ..................................................................
Number of optimal and suboptimal nest sites occupied at differing population densities on Boulder Creek .....................................................
Relationship of winter densities to stream flow in spring .......................
Suggested relationships among major factors affecting size of winter Dipper
population. ..............................................................
Suggested relationships among major factors affecting size of breeding Dipper

population..
..............................................................
Suggested relationships among major factors affecting recruitment of Dippers ....

iv

5
6
8
17
19
21
22
24
25
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INTRODUCTION
The major objective of this study was to answer the basic question: What factors

influence the dynamics of Dipper (Cinch mexicanus)populations? Detailed objectives were: 1) to measure changes in population size, dispersion, and movements; 2) to quantify available resources; 3) to measure impact of social interaction, especially territoriality, on population dynamics; 4) to measure reproductive
success and relate it to other factors, especially territoriality; and 5) to monitor
abiotic factors such as weather and stream flow, and to measure their impact on
population processes.
BACKGROUND
Despite the importance of understanding population dynamics, the problem of
what factors determine sizes of populations is still very much under investigation.
Many hypotheses have been proposed, but most concern only one or two factors,
and no theory has been, or is likely to be, accepted to the exclusion of others
(Watson 1973). For more progress to be made, population studies must become
more holistic and measure the constellations of factors which interact in time and
space to influence population processes (Southwood 1968, Lidicker 1973, Ehrlich
et al. 1975). Field studies on most organisms are unlikely to produce sufficient
relevant data without massive, long-term research programs; even then, results
may be inconclusive (Chitty 1967). Laboratory systems can be simplified and
controlled to the point where clear results are obtained, but these are difficult to
apply to nature.
A search for less complex natural systems should prove useful in clarifying
population processes (Maynard Smith 1974). As an example, intertidal ecosystems
have proven valuable for many types of ecological research (Connell 196 1, 1970;
Frank 1965; Menge and Menge 1974) because the invertebrate inhabitants tend
to be sessile or to move slowly on a two-dimensional surface. Students of vertebrate population ecology have found it difficult to obtain comparable results.
Most vertebrates are relatively mobile (hence opportunistic) and potentially interact with a great many resources, organisms, and environments.
An ideal species for studies of population dynamics would have a number of
characteristics: 1) individual organisms should be easily observed and censused;
2) social behavior should be observable; 3) populations should be large enough
that satisfactory quantities of data can be collected in reasonable time; 4) members
of the population should be individually recognizable, or at least easily marked;
5) the species should have a well-delimited habitat so that an entire population
can be studied; 6) major resources likely to influence the population should be

quantifiable; 7) effects of interspecific competition and predation should either be
quantifiable or not significant; and finally 8) the population should be sedentary
or have quantifiable immigration and emigration. Obviously, no species outside
the laboratory will satisfy all of these criteria, but birds of the Dipper family
(Cinclidae) appear to have a relatively simple ecology and hence are especially
well suited to studies of population dynamics.
ECOLOGYOF DIPPERS
The four species in the Dipper family are allopatric, occurring in Europe and
central Asia (Cinch cinch), eastern Asia and Japan (C. pallasiz], western North


2

STUDIES

IN AVIAN

BIOLOGY

NO. 7

America (C. mexicanus) and South America (C. Ieucocephalus)as far south as
Argentina (Greenway and Vaurie 1958). The range of the American Dipper (C.
mexicanus)extends from Alaska to southern Mexico (Bent 1948, Van Tyne and
Berger 1959). The family is ecologically homogeneous, with all species restricted
to swift, unpolluted, rocky streams. There is only one reference in the literature
to an American Dipper more than a few meters from water, and that was of an
individual flying across a “Y” in a stream (Skinner 1922).
Dippers establish linear breeding territories because of the nature of their habitat, and all activities take place within the territory (type A territory of Nice
194 1). The spatially simple habitat makes it extremely easy to census a population,

map territories, and find individuals without territories. The fact that they so
rarely fly over land makes it easy to capture almost any individual by placing a
net across the stream in its path.
Dippers typically place nests directly over water on ledges of cliffs or bridges
that are inaccessible to predators and sheltered from weather. If such sites are not
available, Dippers may nest in more exposed sites, such as on large rocks or under
tree roots and overhanging banks. Although nests in trees and shrubs away from
water have been reported (Moon 1923, Robson 1956, Balat 1964, Sullivan 1966,
Trochot 1967) they are rare and we did not see any. Such specialized nest-site
requirements make it comparatively easy to find virtually all of the breeding pairs
in a given area. Henderson (1908) and Bakus (1959a) give details of nest construction by C. mexicanus.
Dippers mostly feed on aquatic insect larvae, but occasionally take other invertebrates and small fish (Mitchell 1968, Vader 197 1). Steiger (1940) reports that
they eat some plant material, but Mitchell (1968) does not mention any plant
material in a detailed analysis of 26 stomachs. Although Dippers do flycatch and
glean prey from streamside rocks, most foraging is in water (Sullivan 1973) and
even prey taken out of water are likely to have aquatic larval stages. Thus, Dippers
are totally dependent on the productivity of streams and rivers. This restricted
foraging habitat is more easily sampled for amount of available food than are the
habitats of most terrestrial vertebrates.
Dippers are excellent swimmers and many observers (e.g., Muir 1894) have
been impressed by their ability to forage in water too deep and too swift for
humans to stand upright. Their feet, although large and strong, are not webbed,
and they mainly use their wings when swimming in fast water (Goodge 1959).
Despite their ability to swim, Dippers more frequently wade in the shallows with
their heads submerged, or make short dives into slightly deeper water from perches
on emergent rocks. The quality of an area of stream depends on the stream
substrate as well as on the amount of food. Favorable bottom consists of rubble
(rocks 3-20 cm in size) with many emergent rocks for perching. It is relatively
simple to estimate the percentage of a section of stream covered by rubble and
thus obtain an index of the physical suitability of that section for foraging. In

addition, Dippers’ long, unfeathered tarsi and habit of perching on rocks make it
easy to read color-band combinations.
Many workers describe Dippers as sedentary residents that occasionally make
local altitudinal movements in winter (Bent 1948, Robson 1956, Shooter 1970).
However, some Dipper populations are mobile and make regular flights between
drainages (Jost 1969, present study). There are no reports of regular, long-distance
migrations.


DIPPER

POPULATION

ECOLOGY

3

Dippers also appear to be variably territorial in winter. Some workers suggest
strong territoriality in winter (Skinner 1922, Vogt 1944, Bakus 1959b), while
others report considerable flexibility (e.g., Balat’s 1962 report of males foraging
within 1 m of each other).
There have been a number of good studies covering different aspects of Dipper
natural history. We shall make no attempt to review these further. except as they
pertain to specific population processes. The reader who wishes to know more on
the ecology of this unique group should consult the following: Bent (1948); Hann
(1950); Robson(1956); Bakus (1957, 1959a, b); Balat (1960, 1962, 1964); Hewson
(1967); Haneda and Koshihara (1969); Fuchs (1970): Shooter (1970); Sullivan
(1973). Murrish (1970a, b) reported on interesting physiological adaptations to
temperatures and diving, and Goodge (1959, 1960) discussed locomotion and
vision.

For Dippers, as for most vertebrates, predation and competition are among the
most difficult to quantify of all population processes. Because of Dippers’ alertness,
their open habitat, and the inaccessibility of most nests, we do not feel that
predation is a major cause of mortality for adults or nestlings. Newly fledged
juveniles, however, are more likely to be taken by predators.
Dippers have comparatively few competitors. Belted Kingfishers (Megaccvylc
alcyon) are not common in our study areas (one or two per study area) and are
almost exclusively piscivorous (Bent 1940). Trout are more likely to be competitors of Dippers because of overlap in food (Carlander 1969). Rainbow trout
(Salmo guirdnevz) were most common on our streams (biomasses up to 54 kg/
ha), with much smaller numbers of brown trout (Salmo trutta) and brook trout
(Salvelinusfontinulis)
(J. T. Windell, unpubl. data). Unfortunately, the extent of
niche overlap between trout and Dippers is not known. Data reported by Carlander
(1969) indicate that rainbow trout take a wider variety of foods than Mitchell
(1968) reported for Dippers, but the data on Dippers are comparatively meager.
There are a number of potential differences between the niches of trout and
Dippers, such as preferred water depth, substrate, time of feeding, and proportion
of prey taken as drift (Waters 1962, Lewis 1969, Jenkins 1969, Jenkins et al.
1970, Griffith 1974). However, more data are needed to clarify the extent of
competition between trout and Dippers.
Realizing that Dippers are exceptionally well suited to population studies, we
decided to attempt as complete a study as possible of the dynamics of a Colorado
Front Range Dipper (Cincfus mexicanus unicolor) population. To no one’s surprise, we were not entirely successful. We advance this report in the belief that
our methods, results and organism have heuristic value. In addition to much
intrinsically interesting, basic data on the ecology of Cinclus mexicanus,we have
two general points.
First, population dynamics of even an ecologically simple species are influenced
by many variables. At least eight factors significantly affected our populations
and at least four more remain unstudied. The important factors, actual and potential, ran the gamut from temporal, stochastic, and abiotic phenomena (season,
weather, geology), to biota (food, vegetation, predators) and social interactions

(mating systems, territoriality).
Second, we encourage other ecologists to choose organisms and/or study areas
that, like ours, make holistic studies feasible. Dippers (Cinclidae) are eminently
suited to such investigations and will certainly repay further study.


4

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STUDY

BIOLOGY

NO. 7

AREAS

Field work for this study was conducted in the Front Range of the Rocky
Mountains near Boulder, Colorado. For general discussions and references on the
topography, climate and vegetation ofthis area, see Gregg (1963), Paddock (1964),
and Marr (1967). Dipper populations on two streams, Boulder and South Boulder
Creeks, were selected for intensive study (see Fig. 1).
The two study areas are generally representative of Front Range streams; they
are fast-flowing, clear, rocky-bottomed creeks. Both flow east from headwaters at
3300-4000-m
elevation along the continental divide, dropping rapidly for some
40 km to emerge suddenly from narrow canyons onto the plains at approximately
1650 m. Boulder Creek flows through the town of Boulder, and South Boulder

Creek through the small community of Eldorado Springs before they join and
eventually enter the South Platte River (Fig. 1). Because Dippers require pristine
mountain streams, they do not extend more than a few kilometers onto the plains.
Humans have damaged the habitat by mild pollution and some channelization,
but have also improved it by constructing bridges which serve as excellent Dipper
nest sites, and, on Boulder Creek, by constructing a hydroelectric plant which
keep much of that stream ice-free in winter.
The two principal study sites were divided into 400-m segments, which were
numbered from downstream to the tops of the study areas (49 for Boulder and
23 for South Boulder). Throughout the rest of this paper we will use “segment”
to refer to these divisions of the study sites.
SOUTH BOULDER CREEK STUDY AREA
The South Boulder Creek site extended 9.3 km from the Colorado Department
of Water Resources gauging station at 1920 m elevation down to an irrigation
ditch at 1670 m (Fig. 2). The stream’s drainage basin encloses a total of 308 km2.
The upper 0.5 km of the study area (segments 23-22) has been disturbed by
construction of the Moffat Diversion Dam which backs up a small reservoir for
diversion to the city of Denver. There is ample flow below the dam to maintain
a natural stream environment.
The next 2.6 km (segments 22-16), from the Moffat Dam to South Draw (Fig.
2), is relatively undisturbed. The slope is 2.3%, the substrate is mostly rubble,
and there are many emergent rocks. The banks are extensively lined by willow
(S&X), alder (Alnus), and occasional ponderosa pine (Pinus ponderosa) and narrowleaf cottonwood (Populus angustifolia).
The section from South Draw 1.0 km downstream to Rattlesnake Gulch (segments 16-l 4) has been severely disturbed by flood control channelization for a
small group of houses and a campground. The slope is still gentle (2.0%), but
there is little vegetation along the banks, and the creek bottom is mostly small
rubble with few emergent rocks.
The 0.8 km below Rattlesnake Gulch to just above the town of Eldorado Springs
(segment 14-l 2) is steep (10.0% grade) and narrow, with little quiet water. There
has been some disturbance of the south bank by road construction, but even on

the undisturbed side there is only moderate vegetative cover. The creek bed
probably has always been mostly boulders.
At this point South Boulder Creek emerges from its canyon and for the next


DIPPER

10

0

POPULATION

10

20

ECOLOGY

30

40

50

Kflometers

FIGURE 1. General map of study area. Shaded areas enclose intensive study areas shown in detail
in Figures 2 and 3. (Abbreviations of towns from north to south: Fc, Fort Collins; Es, Estes Park; Lv,
Loveland; Gr, Greeley; Ly, Lyons; Lt, Longmont; El, Eldora; Nd, Nederland; Ep, East Portal; Ro,

Rollinsville; PC, Pinecliff; Ed, Eldorado Springs; Ma, Marshall; Is, Idaho Springs; Gn, Golden; Ka,
Kassler; Dk, Deckers. Reservoirs: 1, Barker Reservoir near Nederland; 2, Gross Reservoir near Eldorado Springs; 3, Cheeseman Reservoir near Deckers.)


STUDIES

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NO. 7

JOHNSON

SOUTH

-/V

0

0.5

I .o

2.0

Kilometers

FIGURE
2. Map of South Boulder Creek study area. (The stream and major tributaries are

represented by solid lines, roads by dashed lines, and intermittent streams and irrigation ditches by
dashed and dotted lines.)

0.8 km (segments 1 l-10) flows through the community of Eldorado Springs.
Despite some dumping of trash and about 200 m of channelization above the
claypit bridge (Fig. 2) the town has relatively little effect on the stream. The
bottom is rubble, with many emergent rocks, and the slope is 3.8%. There are
small thermal springs at the western end of Eldorado Springs which keep a variable
length of stream open and habitable for Dippers in winter.
In the remaining 3.7 km of the study area below the claypit (segments 10-l)


DIPPER

POPULATION

ECOLOGY

7

the slope is 1.6%, the bottom excellent. food abundant, and banks almost completely lined by undisturbed riparian woodland of cottonwood. willow, alder, and
box elder (Acer). There is some residential development along the south bank in
the lowest 1.9 km.
Below the study site, irrigation and civic water supply ditches cause severe
dewatering except during spring runoff. The remaining 9.7-km section. before
South Boulderjoins Boulder Creek (Fig. 1), is increasingly inhospitable for Dippers
because of dewatering in early spring and late summer, channelization, and subdivision construction.
Width of the stream varies from less than 1 m in the narrow canyon to over
15 m in the bottom section. Depth varies from a few centimeters to more than
2 m. Mean daily discharge during the study ranged from 0.08 m3/sec in late

February and early March 197 1 to 12.3 m3/sec on 27 and 28 June 197 1 (Colorado
Department of Water Resources, pers. comm.).
BOULDER CREEK STUDY AREA
The Boulder Creek study area extended 20.0 km from the junction of Middle
and North Boulder Creeks at 2 100 m elevation down to the Boulder sewage plant
outflow at 1600 m (Fig. 3). Area of the drainage basin totals 290 km’. The
vegetation is similar to that of South Boulder Creek. Boulder Creek has no steep
areas comparable to South Boulder Creek and has been more heavily modified
by humans.
The upper 2.7 km from Boulder Falls to Black Tiger Gulch (segments 49-43)
is the steepest, with an average grade of 7.7%. This area has been disturbed
comparatively little, although in places the stream bed was narrowed during road
construction.
The 7.6 km from Black Tiger Gulch to the bridge below the junction with
Fourmile Creek (segments 43-26) is the least disturbed physically. It has a gentle
slope (2.8%) and more rubble substrate than the section above. There is slight
pollution from a septic system below Lost Gulch, but this is rapidly diluted.
The 2.4 km from the bridge below Fourmile Creek to the junction of Arapahoe
Road and Canyon Boulevard (segments 25-18) is slightly steeper (2.9%) and is
severely damaged. Road construction has narrowed the stream bed and filled it
with large boulders, retaining walls have been built to retard bank erosion, and
several large areas have been channelized and have little streamside vegetation.
The first of many irrigation ditches begins dewatering the creek.
Just below the road junction the creek emerges from its canyon and flattens to
a 1.4% grade. The city of Boulder occupies 5.3 km of the stream bank. In the 2.0
km above the Broadway bridge (segments 18-l 4) the creek is in good condition.
It runs through a mixture of residential areas and parks and is relatively undisturbed. Just below the bridge, however, an irrigation ditch may almost completely
dewater the stream in early spring and late summer. In the 3.2 km from Broadway
to the east Arapahoe Road bridge (segments 13-5) Boulder Creek is severely
disturbed by polluted drainage from a gas station just below the ditch, and by

flood-control channelization. For more than half of this stretch there is no streamside vegetation, the bed is bulldozed, and, except during periods of dewatering,
there are few emergent rocks.
In the 1.9 km from the easternmost Arapahoe Road bridge to the sewage outflow


STUDIES

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N. BOULDER CK
UPPER END OF
STUDY AREA

BLACK TIGER GULCH
//

-N

City of
BOULDER

LOWER END OF
STUDY AREA

Kilometers

To Junction with
S. Boulder Creek

FIGURE 3. Map of Boulder Creek study area. (The stream and major tributaries are represented
by solid lines, roads by dashed lines, and intermittent streams and irrigation ditches by dashed and
dotted lines. Note that this map has been divided at Fourmile Creek to conserve space.)


DIPPER

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9

TABLE 1
COMPARISON

OF HABITAT

QUALITY

AND

POPULATION

DENSITY

OF STUDY

AREAS

Study area
Boulder Creek

Mean
Mean
Mean
Mean

width index/segment”
cover index/segment”
bottom index/segment”
food density index/segment”

No. quality 3 nest sites/km=
Breeding density/km
1971
1972
1973
All years:Mean f SD
C+

3.39
2.82
2.67
3.01

0.85
1.12
1.47
0.96
1.18 t 0.26
0.22

South Boulder Creek

3.42
3.17
3.47
17.22 (segments l-23)
5.82 (segments 10-23)
1.86
1.62
1.73
1.40
1.58 i 0.17
0.11

aSee
Methods
b Coefficxnt

section for explanation of indlces.
of variation.

(segments 5-l), Boulder Creek itself is relatively undisturbed. It flows through
riparian woodland and has a canopy of cottonwoods, although there are few shrubs

along the banks because of grazing by cattle. There is a good rubble bottom and
enough groundwater enters the stream bed to maintain some flow even during
severe dewatering. From the sewage outflow to the junction with St. Vrain Creek
(Fig. 1) Boulder Creek is severely polluted, increasingly sandy, and not Dipper
habitat.
Perhaps the most important human influence on Boulder Creek is the Colorado
Public Service Company hydroelectric plant in about the middle of the study area
(segment 30). That plant, which gets its water via a pipeline from Barker Reservoir
(Fig. l), provides power only during periods of peak demand, during which its
discharge may raise the water level of Boulder Creek 0.5 m or more, with a
maximum discharge 5.7 m3/sec (Colo. Public Service Co., pers. comm.). These
rapid fluctuations in water flow keep the stream ice-free below the plant and
provide critical winter habitat that would otherwise be unavailable to Dippers.
The width of Boulder Creek varies from 1.2 m in the upper canyon to over 20
m in the lowest channelized portion. Depth varies from a few centimeters to over
2 m. Mean daily discharge during the study ranged from 0.104 m’/sec on 3 1
December 197 1 to 19.2 m3/sec on 20 June 197 1 (Colo. Dept. Water Resources,
pers. comm.).
COMPARISON OF BOULDER CREEK AND SOUTH BOULDER CREEK STUDY AREAS

In general, South Boulder Creek had better habitat than Boulder Creek. Table
1 contains summaries of width, bottom, cover, and food-density indices for the
two study areas, along with density of good nest sites and density of breeding
birds (see section of Methods for definitions of indices). South Boulder Creek
clearly was better by all of these measures. Note especially that it had densities
of breeding birds that were 34% higher, but only half as variable as those on
Boulder Creek.


10

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NO. 7

OTHER STUDY AREAS
In addition to the intensive study areas on Boulder and South Boulder Creeks,
portions of both streams up to elevations of 3050 m were visited periodically,
especially during the breeding season. Once we discovered that local Dipper populations were more mobile than expected, we made irregular visits to Lefthand,
St. Vrain, and Clear Creeks, to the South Platte River below Deckers, and occasionally to the Big Thompson River, Coal and Ralston Creeks, and many small
streams near the continental divide (Fig. 1).
METHODS
Principal objectives of this study were 1) to describe population dynamics of
the Dipper, especially density, dispersion, territoriality, movements, mortality,
and recruitment: and 2) to relate these to quantified resources and environmental
variables. Methods used for the first objective were relatively standard: banding,
censusing, mapping territories, and monitoring nests. An advantage of studying
Dippers is that these methods are less time-consuming than with most species.
Resulting extra field time and the nature of the species’ habitat and feeding habits
made it possible to quantify resources and various factors of the abiotic environment for the second objective.
Data were collected from 7 February 197 1 to 27 July 1973 on a total of 472
field-days: 306 and 192 days, respectively, for the Boulder and South Boulder
Creek study areas, and 68 days for other areas. Because amount of effort may
affect quantity of various data, several indices of monthly effort were tabulated.
In most cases amount of effort did not correlate with variation in data. Daily
summary maps were prepared, listing observers, areas of stream covered, numbers

and identities of birds seen, and status of nests visited. Information on identified
birds was transcribed onto individual bird data sheets and maps. Data on nest
construction, dates and numbers of eggs, nestlings, and fledglings were tabulated
on individual nest summary sheets.
MAPS AND MEASUREMENTS
Study area maps (used for individual records and summaries) were traced from
United States Geological Survey 7.5-minute topographic maps. Some distance
measurements were made to the nearest 0.1 km on the original topographic sheets
with a measuring wheel. Territories were measured in the field using a 50-m steel
surveyor’s tape. Elevation measurements of nest sites (variable ELEV; see Table
2) were taken directly from topographic maps.
BANDING
Because of the importance of identifying individuals in a study such as this, we
made every effort to band as many Dippers as possible. In all, we banded 558
individuals. Of these, 341 were captured on our study areas and 2 17 at higher
elevations on the study streams or on the nearby drainages of Lefthand Creek,
St. Vrain Creek, and the Big Thompson River. Adults were captured by chasing
them into a mist net stretched across the stream. Nestlings and some females
were hand-captured by climbing to the nests with a ladder or rock-climbing
equipment. Nestlings were banded before 14 days of age, because older nestlings


DIPPER

POPULATION

ECOLOGY

frequently left the nest early when startled. A few fledglings were captured with
a hand net or by hand. All birds were banded with unique combinations of an

aluminum U.S. Fish and Wildlife Service band and various colored plastic bands.
Individual birds will be identified in this paper by the last four digits of the federal
band number.
After banding, birds were weighed and released. Wing length also was measured
in the last spring of field work. Dippers have long, unfeathered tarsi and we could
read band combinations from as far as 30 m with 10X binoculars. Few returns
were made through the U.S. Fish and Wildlife Service Bird Banding Office and
all but five sightings used in this report were made by personnel working on the
project and familiar with the color scheme. For each banded bird an individual
data sheet and map were kept, and all subsequent sightings were recorded, along
with notes on behavior, mates, breeding, plumage, etc.
DETERMINATION OF SEX AND AGE
Although Dippers appear monomorphic, only females incubate (Jourdain 1938,
Bakus 1959a, Haneda and Koshihara 1969) and males have longer wings than
females (BalBt 1964; Andersson and Wester 197 1; Price, unpubl. data). Prior to
spring 1973, however, we were not aware of the dimorphism in wing length and
could sex birds only by observing a brood patch or incubation behavior during
the breeding season.
No method is known for aging Dippers after they complete their postjuvenal
molt. When ages were used in analysis of factors affecting territory size and fledging
success (variables FEMAGE, MALEAGE),
the following scheme was used: breeding individuals banded as nestlings or juveniles were given their true age in years.
From these individuals, a mean was calculated for each sex. Birds of unknown
age when banded were assigned an age equal to the mean for their sex. Unknowns
observed again in subsequent years were assigned ages equal to the mean plus
one, or mean plus two years. Although this procedure probably underestimated
the mean age of unknown birds, we believe it made the best use of our data. Our
sample of birds with known ages was too small to evaluate effects of age on
territory size and fledging success. Since age may well be an important variable
we decided that even an underestimate was useful.

CENSUSING
Throughout the study a complete census was attempted once a month by two
or more observers walking the length of each intensive study area. When possible,
at least one observer waded. Since a census of both study areas usually required
7-10 days, censuses were not done during the breeding season when other data
were needed and the location of each breeding pair was known. Certainly we spent
enough time in the study areas during breeding seasons to have found any nonterritorial birds.
Dippers are more easily censused than most birds, but there were a number of
sources of error associated with this technique. The major difficulty was that some
birds remained motionless in hiding until the observers passed. This was especially
common in winter when there were air pockets under shelf ice, and in spring when
high water made it difficult to see and hear birds (see Bakus 1957 and 1959b for
a more detailed discussion). By working down the stream in pairs, throwing rocks


12

STUDIES

IN AVIAN

BIOLOGY

NO. 7

into dense bushes and by ice ledges, pounding on thick ice with poles, and sending
one observer back after unidentified birds that flew past, it was possible to see
the vast majority of the population. Thus, most inaccuracies mentioned by Bakus
were avoided or minimized, and censuses were, to the best of our ability, “true
censuses,” not “sampling estimates” (Smith 1966).

The number of birds seen on each stream segment was recorded as the variable
NUMBIRDS
for use in analysis of dispersion. Because few censuses were taken
during breeding seasons, an estimate of breeding season density per stream segment was calculated by the formula:

D, =

c V,/A,)P,

/=I.2

where D, was the estimated density in segment i (ESTBIRDS);
T, was the total
number of segments occupied by the territory of femalej whose territory included
segment i; A, was the number of adults in the territory of female j (i.e., 2.0 for
monogamous and 1.5 for polygynous territories); and P,j was the proportion of
segment i occupied by the territory of female j. No segment was ever occupied
by more than two females. Our use of this equation assumes: 1) that polygynous
males divided their time equally between the territories of two females, and 2)
that all parts of a territory were utilized equally. Although it is probable that
neither of these assumptions was completely satisfied, we believe that the above
formula provides the best possible estimate of ecological density of breeding
Dippers. Indeed, these calculations of breeding bird density per 400-m segment
probably were more realistic than estimates based upon censuses. Breeding birds
were, in effect, “spread” over the sections of stream they used, rather than being
placed in a segment where they happened to be seen on a census.
Peripheral areas off the main study areas (see section on Other Study Areas)
were spot-checked in nonbreeding seasons, but these data were incomplete. During
breeding seasons only potential nesting sites were examined for evidence of breeding activity. Because of the restricted nest site requirements of this species, censuses
off the main study areas were reasonably complete for breeding birds, but not for

transients.
DETERMINATIONOFTERRITORY BOUNDARIES
Most students of Dippers have used chases to determine territory boundaries
(e.g., Vogt 1944, Robson 1956, Bakus 1959b, Balat 1962, Sullivan 1973, Sunquist
1976). This method assumes that the birds will go to an end of their territories
before turning, but Bakus’ (1959b) data and our own indicate that this is not
always true. During the first few days of territory establishment, some birds would
consistently turn in the same area, but others were never consistent. Later, even
individuals that had gone to the boundaries turned at different points, possibly
because they were familiar with places to hide within the territory or had become
habituated to the chase situation. The best data on the location of territory boundaries came from observing territorial encounters between neighboring birds.
Whenever possible in this study, two observers chased birds together to determine
where boundaries lay. If this was not possible, the boundary was set where the
birds turned around, provided this was consistent two or more times early in the
season and neighbors independently turned in approximately the same place.


DIPPER

POPULATION

ECOLOGY

13

Encounters between territory holders and wandering individuals were not good
indicators of boundaries. Territory owners frequently landed before reaching their
boundary and sang while the intruder kept flying. When none of these techniques
worked, especially for isolated, open-ended territories without neighbors, only the
observed home range (Burt 1943) was mapped. Territory sizes for females were

recorded as the variable FEMTRSIZ
for use in statistical analyses. Territoryboundary data for the Boulder Creek study area in 197 1 were inadequate by these
guidelines and were not used in statistical analyses.
MEASURESOFHABITATQUALITY
Because one objective of this study was as complete an assessment as possible
of the components of habitat suitability, a number of additional variables were
quantified. The names and definitions of the variables used in analyses are shown
in Table 2, and are described below.

Food availability
Food availability was assessed using a Surber sampler (Hynes 1970) to estimate
biomass of benthic invertebrates. On the Boulder Creek study area, 1 l-l 6 stations
were sampled in winter 197 l-l 972 (February), summer 1972 (July), winter 19721973 (December), and in spring 1973 (April). Unfortunately, mild spring weather
in early 1972 prevented a spring sample in that year and we used the spring food
data from 1973 in analyzing all three years’ data. In the same months, 9-l 3 stations
were sampled on the South Boulder Creek study area. The sampler was handmade
of anodized aluminum and had a sample area of 0.1 m2; the net had a mesh with
nine threads per centimeter. Every effort was made to catch organisms on and
under rocks, but not to sample deeply buried organisms which would be less likely
to be available to Dippers. Six such samples were taken at each station (or three
if insects and debris were very abundant) and collected material was preserved
in 95% ethanol. Later, organisms larger than 1 mm (mostly insect larvae) were
separated by hand. Samples were then air-dried for 5 min and weighed to the
nearest 0.01 g. In calculating biomass, each set of six samples from a station was
considered to be of 0.5 m2 to compensate for losses in sampling, as suggested by
Dr. R. W. Pennak (pers. comm.). Because areas with rubble bottom are more
productive than areas with boulders, gravel, sand, or silt (Pennak and Van Gerpen
1947) samples were not taken at random. Rather, they were taken in shallow (550 cm deep) areas of rubble that experience had indicated were suitable for Dipper
foraging. Quantification of relative amounts of rubble in different parts of the
study areas is discussed below under bottom-quality index.

Organisms were not sorted into taxa or size classes, nor were stomach samples
taken. Work by Mitchell (1968) Thut (1970) and Vader (197 1) indicates that
Dippers will take almost any animals (within a broad size range) available in the
stream. Nor did we sample aerial or terrestrial prey, which Sullivan (1973) found
to be the objects of approximately 20% of Dipper foraging maneuvers in spring
and summer. Because many insects in the air and on streamside rocks have aquatic
larvae, we considered this to be an insignificant source of error.
There is a large body of literature on inaccuracies of available techniques for
sampling stream benthos (see Hynes 1970 for a general discussion and references).
Our measurements were not intended to be accurate determinations of total ben-


STUDIES

14

IN AVIAN
TABLE

LIST OFVARIABLE

BIOLOGY

NO. 7

2

NAMES USED IN THE ANALYSES~

A. Variable names used in analysis of dispersion

BOTM
COVR
ESTBIRDS
ICE
INTFOOD
NSQDIST
NUMBIRDS
NUMBRIDG
REALFOOD
SITEQUAL
TOTSITQL
WIDTH

= Bottom quality index of a stream segment
= Index of percent of stream bank in a segment covered by rocks, vegetation or
other things suitable for hiding Dippers
= Estimated density of breeding Dippers utilizing a segment
= Index of ice cover
= Interpolated food index for a stream segment
= Index of quality and distance of nest sites in or near a stream segment
= Number of Dippers seen in a segment on a census
= Number of bridges in a segment
= Measured stream insect biomass in a segment
= Index of nest site quality
= Sum of SITEQUAL
of all nests sites in a segment
= Width index of a stream segment

B. Variable names used in analysis of territory size and reproductive success
CLCHNUM

DICUP
DIDOME
DREGG
D8FLEDG
DLHATCH
D8INCUB
D8START
ELEV
FEMAGE
FEMTRSIZ
FLOB4CON
FLONSTL
MALEAGE
MEANFOOD
NOEGGS
NOFLEDG
NONESTL
OPNENDS
POLYGYNY
SITEHITE
TOTAGE
TOTFOOD
TPTNINC
TPTNNSTL
XMNTINC
XMNTNSTL
XPTNINC
XPTNNSTL

=

=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=

Clutch number, i.e., 1st 2nd, replacement

Date inner nest cup was completed; days from 1 January
Date nest dome was completed; days from 1 January
Date first egg was laid; days from 1 January
Date nestlings first left the nest; days from 1 January
Date eggs hatched; days from 1 January
Date incubation began; days from 1 January
Date nest construction began; days from 1 January
Elevation of nest site above sea level
Age of female parent
Size of female’s territory
Mean stream flow during the week before D8START
Mean stream flow during the nestling period
Age of male parent
Mean of interpolated 1973 food samples at 100-m intervals in territory
Number of eggs in completed clutch
Number of nestlings fledged
Number of nestlings
Presence of territory boundaries not adjacent to a neighboring territory
Presence or absence of polygynous mate
Height of nest site above water surface
Sum of FEMAGE + MALEAGE
Product of MEANFOOD
x FEMTRSIZ
Total precipitation during incubation
Total precipitation during nestling period
Mean minimum daily temperature during incubation
Mean minimum daily temperature during nestling period
Mean precipitation per storm during incubation
Mean precipitation per storm during nestling period

thic biomass or of total Dipper food, but rather to be reasonably reliable indices
of food availability in different portions of the study areas. A number of samples
were replicated after a few days and found to be within 1 g of one another.
Food sample data were plotted against their locations and recorded as the
variable REALFOOD
for each stream segment from which a sample was taken.


DIPPER

POPULATION

ECOLOGY

15

Linear interpolations were made between sample points. In analyzing effects of
food availability on dispersion we also took the value of the food graph in the
middle of each 400-m stream segment to be representative of that segment and
recorded it as the variable INTFOOD.
For analyses of relationships between food
availability and territory size and placement. we mapped territories along the food
graph. At 100-m intervals in each territory the values of the food graph were
averaged to obtain an estimate of mean food density in each territory (variable
MEANFOOD).

Nest sites
The numbers and qualities of nest sites in each segment were determined.
Quality of each nest site (abbreviated SITEQUAL)
was graded from 1 (poor) to

3 (excellent) on the basis of four criteria: height above water, ledge width. presence
of a sheltering overhang, and security from predators. Quality 1 sites were within
1 m of water level in early April or were easily accessible to predators. Quality 2
sites were high and inaccessible, but lacked a sheltering overhang. or the ledge
was less than 10 cm wide. To be rated as quality 3, a site had to satisfy all four
criteria.
If Dippers are attracted to nest sites and tend to spend time near them, the
probability of our seeing a bird should vary directly with the quality of the nearest
nest site and inversely with its distance. An index ofnest site quality and dispersion
(abbreviated NSQDIST) was calculated for each segment by the formula:
I, = q,/d, + qz/dz,
in which I, was the index (NSQDIST)
of the ith segment, q, and q2 were the
qualities of the nearest nest sites up and downstream, respectively, and d, and d2
were the distances in number of 400-m segments to the nearest nest sites up- and
downstream. To avoid division by zero, we gave segments containing a nest site
a distance of one; segments lacking a site but adjacent to one with a site were
given a distance of two, etc.

Stream quality
To measure additional aspects of stream quality, the center of each stream
segment was marked on a map and visited in random sequence by the same two
observers. The observers each walked up- and then downstream 100 m from the
center and independently rated width, bottom, and cover. Width (WIDTH)
of
bed (not water) was graded from 1 (less than 4 m) to 6 (more than 20 m). Bottom
quality (BOTM) was rated subjectively from 1 (very poor) to 5 (very good) on
the basis of amount of bed covered by rubble (rocks 3-20 cm in size). bed profile,
depth, and number of large rocks available for perching. Amount of cover (COVR,
i.e., large rocks, bridges, and vegetation) along the banks was graded 1 (no cover),

2 (less than 10% cover), 3 (lo-to-50% cover), or 4 (more than 50% cover). During
winter censuses the amount of ice in each 400-m segment of stream (variable
ICE) was rated from 0 (no ice) to 3 (very little open water).
For each segment, the mean score on each variable (WIDTH,
BOTM, COVR,
and ICE) was taken as representative of the entire segment, and used as an index
in statistical analyses. A number of other parameters and rating schemes were
evaluated and this sytem proved most reliable (interobserver correlation = 0.83).


STUDIES

16

IN AVIAN

BIOLOGY

NO. 7

Depth could not be reliably rated; because of significant daily fluctuations, many
measurements would have been needed at each point and it was judged not worth
the time required. Also, general water depth was a component of the bottom
evaluation.

Stream flow
Data on mean daily stream discharge were obtained from the Colo. Dept. Water
Resources. These data were gathered from gauging stations located just above the
campground on South Boulder Creek (Fig. 2) and just below the hydroelectric
plant on Boulder Creek (Fig. 3). For each brood, mean stream flow during the

week before nest construction started (FLOB4CON) and mean stream flow during
the nestling period (FLONSTL) were recorded and used in analyses of reproductive
success.

Weather
Data on daily precipitation and daily maximum and minimum temperature
were obtained from published U.S. Weather Bureau records for the city of Boulder
(U.S. Dept. Commerce, 1971-1973). Although microclimate on the study areas
certainly varied from the reported Boulder figures, no better data were available.
For analysis of reproductive success, additional variables were computed: total
precipitation during incubation (TPTNINC)
and nestling period (TPTNNSTL),
mean minimum temperatures during incubation (XMNTINC)
and nestling period
(XMNTNSTL),
and mean precipitation per storm during incubation (XPTNINC)
and nestling period (XPTNNSTL).
STATISTICAL ANALYSES
Correlation analysis was used extensively in this study. In analysis of dispersion,
data on density of Dippers and data on environmental variables for each of the
72 stream segments in each census were punched onto Hollerith cards for input
to computer programs. Similarly, pertinent data on each clutch of eggs laid in our
study areas were punched onto cards for analysis of territoriality and nesting
success. Names and definitions of variables used in these analyses are listed in
Table 2. The principal programs utilized were BMD-02R (Dixon 197 1) and various SPSS programs (Nie et al. 1975).
ANNUAL

CYCLE

IN THE COLORADO

FRONT

RANGE

A brief survey of the annual climatic cycle and its effects on Dipper populations
is useful at this point as an introduction to the ecology of the species in our area.
CLIMATE
The climate of the Boulder area is a continental one, with great variations, both
diurnal and annual, in temperature and rainfall (Paddock 1964). Figure 4 shows
mean monthly temperature and total monthly precipitation in the town of Boulder,
and total monthly runoff of Boulder Creek during the study.
Daily temperatures fluctuated an average of 15°C and variations of more than
22°C were not uncommon. Average precipitation was 4’72 mm per year, but was
highly variable, with an average monthly deviation of 25 mm from 30-year means
during the study period. The mean annual discharge of Boulder Creek over 63


DIPPER

POPULATION

17

ECOLOGY

A. Mean Monthly Temperature
7

20-

y

IO-

O-

J
ONDIJFMAMJJASONDIJFMAMJJASONDJFMAMJJA

B Total Preclpltation Per Month
150

C. Total Runoff Per Month

01

’
ONDJFMAMJJASONDJFMAMJJASOND(JFMAMJJA
1970

1971

1972

1973

FIGURE 4. Variation of environmental factors in Boulder, Colorado. (Dashed lines show 30-year
means, 1930-1960; solid lines, data collected during this study, 1971-1973. Sources: A and B, U.S.
Dept. Commerce; C, Colo. Dept. Water Resources.)

years of records has been 8.1 X 10’ m3, with a mean rate of flow 2.6 m3/sec (Colo.
Dept. Water Resources, pers. comm.). Figures for South Boulder Creek are comparable, although more variable. Both streams usually were partly frozen from
middle or late December until mid-February.
These average figures do not give a realistic impression of the often extreme
environmental fluctuations faced by Dippers. For example, May 1969 was wetter
than average (220 mm total precipitation versus a mean of 85 mm), and 87% of
the precipitation fell from 3 to 8 May. This storm increased flow in Boulder Creek
from 1.0 m3/sec on 1 May to 25.9 m3/sec on 7 May, and in South Boulder Creek
from 1.7 m3/sec to 3 1.7 m3/sec. Flood damage along both streams was considerable, and effects on the Dipper population undoubtedly were drastic (M. Whitney, pers. comm.). Temperature also may fluctuate greatly. The winter of 19721973 was unusually severe, with mean monthly temperature falling below 30year averages in November, December, and January by 4.1”C, 4.4”C, and 2.o”C,
respectively (Fig. 4). One 12-day period in December 1972 had a mean daily
maximum temperature of -20°C. The effects of extreme changes in weather are
discussed in more detail in the section on survival and productivity.
It is difficult to compare the annual climatic cycle in Boulder with those of other
Dipper habitats. Dippers live in mountainous areas characterized by large differ-


STUDIES

18

CONTINENTALITY

mexicanus
mexicanus
mexicanus
cinch
cinch
cinclus
cinclus

cinch
cinclus
cinch

INDICES

AND

Missoula, Mont., USA
Missoula, Mont., USA
Boulder, Colo., USA
Westmoreland, England
Banffshire, Scotland
Peak Dist. Natl. Park,
Derbyshire, England
Bern, West Germany
Bmo, Czechoslovakia
Fulda, West Germany
Basle, Switzerland

IN AVIAN

NO. 7

BIOLOGY

TABLE 3
ELEVATIONS OFSTUDIES OF DIPPER POPULATIONS

975-l 220

<975-1220+
1600-2100
I SO-550
ndd

33b
33b
37b
Cl@

Bakus (1957, 1959a, b)
Sullivan (1973)
Present study
Robson (1956)
Hewson (1967, 1969)

90-370
nd
240-340
200-S 10
ca. 270

110’
20-30c
25-30‘
15-2@
20-25c

Shooter (1970)

Vogt ( 1944)
Balat (1960, 1962, 1964)
Jost (1969, 1970)
Fuchs (1970)

=

Index = (1.7 X (.d’sm L))
20.4, where A = annual temperature range (“C) and L = lautude angle (Barry and Chorley 1970).
C
‘ alculated
from data I” U.S. Dept. Commerce (1964. 1965).
’ Estimated from Barry and Chorlq (1970, Rg. 5. I).
d nd = data not avadable.

ences in precipitation and temperature over short distances (Barry and Chorley
1970). However, because published data on the ecology of Dippers frequently
appear contradictory, it is necessary to attempt comparisons. Continental climates
are characterized by a short time lag between maxima and minima of solar insolation and corresponding maxima and minima of surface temperatures (i.e.,
rapid spring thaws and fall freezes), as well as great annual and diurnal temperature
fluctuations. Climatologists have formulated indices of continentality which can
be used in comparing different areas (Barry and Chorley 1970). Table 3 shows
such indices, along with the elevations of some areas where Dippers have been
studied. Other factors being equal, we would expect areas at high elevations and
those with high indices to have less favorable and more variable climates. By
either of these measures the Boulder climate is severe.
DIPPERS

As early as the third week in February, individuals that had wintered in areas
of open water with suitable breeding habitat began to court and establish territories

on their wintering grounds. As the ice melted, nonwintering birds arrived and
also attempted to establish territories and find mates. Birds unsuccessful in establishing territories continued to move until they left our study areas.
Both males and females defended territories, although females appeared to
choose the actual nest sites. Females performed most of the nest construction,
which began l-2 weeks after territory defense. Nest sites and construction followed
the usual cinclid pattern, except that good sites were abundant in our areas and
no nests were seen on sites other than cliffs, bridges, and large boulders.
In the three years of our study there was considerable variation in the timing
of breeding (see Fig. 5). On the lower parts of the study areas egg laying probably
began in early to mid-April in most years, although the start of laying varied from
mid-March in 1972 to early May in 1973. From a comparison of Figures 4 and
5 it is clear that Dippers returned to breeding areas and initiated courtship well


DIPPER

POPULATION

19

ECOLOGY

20

1973

1
15-

IO-

5-

MARCH

APRIL

MAY

JUNE

JULY

AUGUST

I

s
w
st
s
5
z
cn
y
:!
3
0
z

n=34

I5

IO

5

0
MARCH

APRIL

MAY

JUNE

JULY

AUGUST

6

1971
n=24

15

IO

5

0
MARCH

APRIL

MAY

JUNE

JULY

AUGUST

MONTH

FIGURE 5. Timing and number of clutches being incubated, 197 l-l 973. (First and replacement
clutches are represented by dotted bars; second clutches by bars with diagonal lines.)

before the peak of spring runoff in May and June. Especially in 1972, birds began
to appear on the study areas in January even before temperatures rose. The 1973
breeding season was anomalous in this respect, perhaps because of the exceptionally severe winter.
It is adaptive for Dippers to start breeding early because the heavy spring runoff
in May drastically reduces food availability (Mecom 1969). While it is true that
this means most pairs will be feeding young during the runoff, it is equally true
that there would be no more food later in the summer (Figs. 9, 10; Mecom 1969).
Egg formation by female birds is energetically expensive (Kendeigh 1963, ElWailly 1966) and the early start means that most clutches are laid before runoff
starts. While incubation also utilizes energy (Kendeigh 1963) the “oven-like,”
insulated nest which Dippers build is well adapted to reduce heat loss to a minimum. Because of their stringent nest site requirements, suitable nest sites may

often be in short supply. It is probable that there has been selection for defense
of territories and nest sites by Dippers as soon as ice melts.
Despite this apparent selection for early breeding, winter and spring weather


20

STUDIES

IN AVIAN

BIOLOGY

NO. 7

did appear to affect the start of breeding. Temperatures in February and March
1972 were unusually warm, and incubation started almost a month earlier than
in 197 1 when temperatures were close to the 30-year means. Temperature and
precipitation were again close to normal in February and March of 1973, but
incubation did not start until May. It is possible that many birds were in poor
condition following the severe winter of 1972-1973 and needed more time to
come into breeding condition. Our weight data indicate that in the first four
months of 1973, birds averaged 4% lighter than in 1972 (1973 mean = 56.2 g,
y1= 25; 1972 mean = 58.5 g, y2= 31). While this difference was not statistically
significant, these data suggest that adults surviving the winter of 1972- 1973 were
in poor condition.
Dippers laid one egg per day until their clutches were complete (usually four
or five eggs), after which incubation began. The females incubated alone for about
16 days. Although males took no part in incubation, they occasionally fed the
females. Clutches of second, polygynous females (Price and Bock 1973) usually

were started during laying or incubation of the first females’ broods. After eggs
hatched, both male and female fed the young for 20-30 days. On the average,
fledging occurred 25.4 days after hatching (n = 51). After a first brood fledged,
about 40% of adults started second broods. Length of breeding season was important in determining the number of second broods (Fig. 5). No second broods
were seen above approximately 1830 m elevation, although we did see replacement
broods.
After fledging and being fed for from a few days to two weeks, juveniles dispersed, with many crossing over drainage divides to other streams. Most adults
left their territories after breeding and moved upstream, with some changing
drainages during the summer. During this period in August, adults, but not juveniles, underwent a synchronous molt of flight feathers and could not fly for 514 days (Balat 1960; Sullivan 1965, 1973).
Beginning in late August and September, banded birds started to reappear on
our study areas, along with unbanded individuals. Numbers increased into October, then declined in November and December. It is unclear where most of
these birds went; many probably wandered in search of open water.
By mid- to late December most streams had frozen and the only habitat available, aside from small holes, was to be found in the foothills and high plains. On
Boulder Creek the area below the hydroelectric plant (Fig. 3) remained open. On
South Boulder Creek a variable length of stream, sometimes less than 1.5 km,
was kept open by thermal springs. Since Boulder and South Boulder Creeks drain
290 km2 and 308 km2 areas, respectively, there was severe compression of the
population in winter.
Contrary to other reports (Vogt 1944, Bakus 1959b, Hewson 1967, Sullivan
1973) Dippers on our study areas were not clearly territorial in winter. Although
there was much agonistic behavior, there was no clearcut defense of a given space
such as occurred during the breeding season. Individuals often exhibited day-today movements and left the study areas for a month or more.
In January and February the number of birds began to increase again as the
breeding season approached. Individuals seen the previous fall commonly returned, along with large numbers of unbanded birds, and attempted to establish
territories.